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GEOSPHERE Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges and the western Transverse GEOSPHERE, v. 17 no. 3 Ranges in California from reconstruction and luminescence dating https://doi.org/10.1130/GES02274.1

14 figures; 2 tables of the Orcutt Formation

CORRESPONDENCE: [email protected] Ian S. McGregor and Nathan W. Onderdonk Department of Geological Sciences, California State University–Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90804, USA

CITATION: McGregor, I.S., and Onderdonk, N.W., 2021, Late Pleistocene rates of rock uplift and faulting at the boundary between the southern Coast Ranges ABSTRACT consistent with models that attribute shortening across the Santa Maria Basin and the western Transverse Ranges in California from to accommodation of clockwise rotation of the western Transverse Ranges and reconstruction and luminescence dating of the Orcutt Formation: Geosphere, v. 17, no. 3, p. 932–956, https:// The western Transverse Ranges and southern Coast Ranges of California suggest that rotation has continued into late Quaternary time. doi.org/10.1130/GES02274.1. are lithologically similar but have very different styles and rates of Quaternary deformation. The western Transverse Ranges are deformed by west-trending Science Editor: Andrea Hampel folds and reverse faults with fast rates of Quaternary fault slip (1–11 mm/yr) ■■ INTRODUCTION Associate Editor: Jeff Lee and uplift (1–7 mm/yr). The southern Coast Ranges, however, are primarily deformed by northwest-trending folds and right-lateral strike-slip faults with The Coast Ranges of California are deformed by northwest-striking faults Received 15 April 2020 Revision received 16 November 2020 much slower slip rates (3 mm/yr or less) and uplift rates (<1 mm/yr). Faults and folds that accommodate active transpression along the North American– Accepted 20 January 2021 and folds at the boundary between these two structural domains exhibit Pacific plate boundary. This northwest structural grain is truncated to the south geometric and kinematic characteristics of both domains, but little is known by west-striking faults and folds that accommodate north-south shortening in Published online 24 March 2021 about the rate of Quaternary deformation along the boundary. the western Transverse Ranges (Fig. 1). The boundary between these distinct We used a late Pleistocene sedimentary deposit, the Orcutt Formation, as tectonic domains is a diffuse zone ~15–20 km across that facilitates differential a marker to characterize deformation within the boundary zone over the past movement between the two domains. The boundary zone has experienced 120 k.y. The Orcutt Formation is a fluvial deposit in the Santa Maria Basin that historic seismicity and large-magnitude earthquake events, but very little is formed during regional planation by a broad fluvial system that graded into known about the amount of Quaternary displacement on the major faults, a shoreline platform at the coast. We used post-infrared–infrared-stimulated slip rates, or uplift rates. This information is critical for understanding the luminescence (pIR-IRSL) dating to determine that the Orcutt Formation was regional tectonics and topographic development, as well as earthquake risk deposited between 119 ± 8 and 85 ± 6 ka, coincident with oxygen isotope for the local population and critical facilities in the region, such as the Diablo stages 5e-a paleo–sea-level highstands and regional depositional events. The Canyon nuclear power plant and Vandenberg Air Force Base. deformed Orcutt basal surface closely follows the present-day topography Quantitative description of landforms and young deposits is needed to inter- of the Santa Maria Basin and is folded by northwest-trending anticlines that pret the history of Quaternary landscape evolution and faulting (e.g., Bull, 1985; are a combination of fault-propagation and fault-bend-folding controlled by Kamp and Owen, 2012). Measurements of topographic development and fault deeper thrust faults. Reconstructions of the Orcutt basal surface and forward slip are often dependent on localized deposits, such as fluvial terraces or alluvial modeling of balanced cross sections across the study area allowed us to mea- fans, which can be used to bracket the timing and magnitude of offset and uplift sure rock uplift rates and fault slip rates. Rock uplift rates at the crests of two along individual faults. However, if regionally extensive Quaternary deposits are major anticlinoria are 0.9–4.9 mm/yr, and the dip-slip rate along the blind fault present, deformation over a larger area can be analyzed with a single marker, system that underlies these folds is 5.6–6.7 mm/yr. These rates are similar to which provides a more complete picture of tectonic history with less uncertainty those reported from the Ventura area to the southeast and indicate that the in correlation between local deposits (e.g., DeVecchio et al., 2012). We illustrate relatively high rates of deformation in the western Transverse Ranges are also this approach by using the Orcutt Formation, a regionally extensive late Qua- present along the northern boundary zone. The deformation style and rates are ternary sedimentary deposit, as a marker for investigating recent topographic This paper is published under the terms of the growth and the structures controlling this growth at the boundary between the CC‑BY-NC license. Nathan Onderdonk https://orcid.org/0000-0001-7893-061X southern Coast Ranges and western Transverse Ranges in California.

GEOSPHERE | Volume 17 | Number 3 McGregor and Onderdonk | Quaternary uplift and fault slip rates in the boundary zone between the Coast Ranges and Transverse Ranges Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/932/5319294/932.pdf 932 by guest on 28 September 2021 Research Paper

Santa Lucia Mountains Nacimiento F. 120° 30’ W 119° 30’

North America

Rinconada F. N SF California

Los Osos F. San Andreas F. LA San Luis Range Pt. Buchon San Luis Obispo ! Pacific Ocean West Huasna F.

SOUTHERN COAST RANGES 35° 35° N S South Cuyama F. a n

Santa Maria R Pt. Sal Foxen Canyon F. a fa e l CHFZ Casmalia Hills M Hosgri F.Z. LHF o u n ta in s Santa Maria Basin Ozena F. Little Pine F. Vandenberg LAF AFB Purisima Hills West Big Pine F. BF Santa Ynez River F. Pine Mtn F. Honda F. Solvang Pt Arguello

Santa Ynez F. Area of Figure 2 Sant Santa Ynez F. a Ynez Mountains WESTERN TRANSVERSE RANGES Pt Conception Santa Barbara SCF Santa Barbara Channel RMF Pacific

PP- VF Ventura Ocean

0 5 10 20 Kilometers

119° 30’ 34° 120° 30’ 34°

Figure 1. Location map of the western Transverse Ranges and southern Coast Ranges showing topography, main geographic features, and faults (red lines). Fault abbreviations include: CHFZ—Casmalia Hills fault zone, LAF—Los Alamos fault, BF—Baseline fault, LHF—Lions Head fault, RMF—Red Mountain fault, SCF—San Cayetano fault, PP-VF—Pitas Point–Ventura fault. Coloring of topography shows relative elevation across the region with light green shades in lower elevation and orange shades in the higher elevations. Other abbreviations: AFB—Air Force Base, F.—fault, Pt.—Point, F.Z.—fault zone. Inset map shows location within California with the cities of Los Angeles (LA) and San Francisco (SF) for reference.

The western half of the boundary zone is a region of low hills and coastal (Page and Engebretson, 1984; Zoback et al., 1987; Hauksson, 1990), initiated plains called the Santa Maria Basin, which is bounded by the Santa Ynez high rates of shortening across the western Transverse Ranges, and caused Mountains on the south and the Santa Maria River Valley on the north (Figs. 1 inversion and uplift of the rocks in the Santa Maria Basin. Shortening and and 2). The Santa Maria Basin formed during Miocene extension and has since uplift in the Santa Maria Basin are accommodated by a system of folds, Mio- been inverted by thrust faults and folds during Pliocene to present shortening. cene normal faults reactivated with reverse slip, and low-angle thrust faults Retrodeformable cross sections and stratigraphic correlations of Miocene and (Namson and Davis, 1990; Gutiérrez-Alonso and Gross, 1997). Shortening in Pliocene horizons across the basin have been used to estimate fault geome- the Santa Maria Basin may have accommodated differential rotation between tries and total convergence (Krammes and Curran, 1959; Namson and Davis, the western Transverse Ranges and southern Coast Ranges (e.g., Lettis et al., 1990; Clark, 1990; Seeber and Sorlien, 2000). However, these studies did not 2004) as well as the southward decrease in slip on the Hosgri fault and its address the Quaternary deformation. We addressed this lack of knowledge termination offshore of Point Arguello (e.g., Sorlien et al., 1999; Dickinson et using the Orcutt Formation to document late Pleistocene deformation and al., 2005). Shortening has continued into Quaternary time, but the amount tectonic history. The Orcutt Formation is a predominantly fluvial, regionally and rate of Quaternary deformation have not been well studied. Long-term extensive unit that was deposited on a low-relief surface that existed between slip rates on the southern Hosgri fault offshore are around 2 mm/yr (Sorlien et the Santa Ynez and San Rafael Mountains (Fig. 2; Woodring and Bramlette, al., 1999), but there are no published Quaternary slip rates on faults within or 1950; Worts, 1951; Muir, 1964; Dibblee and Ehrenspeck, 1989; Clark, 1990). The at the boundaries of the Santa Maria Basin. The only Quaternary uplift rates Orcutt Formation has been folded, lifted, and faulted across structures in the come from unpublished master’s theses that estimated rates of ~1 mm/yr in Casmalia and Purisima Hills, providing a rare and unique opportunity to assess the central and eastern Santa Maria Basin using fluvial terraces (Farris, 2017; multiple faults and folds with a single deposit, quantify and characterize defor- Tyler, 2013), and 0.012 mm/yr to 0.34 mm/yr from elevated marine terraces at mation, and document the timing and rate of topographic evolution. In this Point Sal (Clark, 1993). study, we present nine infrared-stimulated luminescence (IRSL) dates as the first numerical ages for the Orcutt Formation. We used these dates, along with reconstructions of the Orcutt basal surface and forward modeling of balanced Geology of the Santa Maria Basin cross sections, to make measurements of late Pleistocene rock uplift rates across two anticlinoria and model slip rates on the underlying faults. Stratigraphy

The stratigraphy of the Santa Maria Basin consists of Miocene and younger ■■ GEOLOGIC SETTING sedimentary rocks that overlie metamorphic basement (Woodring and Bram- lette, 1950; Clark, 1990). The basement rocks belong to the Mesozoic Franciscan Regional Tectonic Context Formation, which is an accretionary wedge complex developed during sub- duction along the western edge of North America prior to development of The Santa Maria Basin is located within the transform plate boundary zone the modern transform plate boundary. These rocks are only exposed in a few between the Pacific plate and the North American plate. During a change from locations at the margins of the Santa Maria Basin where they have been lifted a convergent margin to the San Andreas transform system in Neogene time, to the surface by reverse faults that now bound the inverted basin (Woodring broad dextral transtension across the plate boundary resulted in clockwise and Bramlette, 1950). The entire Late Cretaceous to Eocene forearc sedimentary rotation of large crustal blocks and the formation of fault-bounded basins, sequence that overlies the Franciscan basement rocks in the southern Coast including the Santa Maria Basin, Los Angeles Basin, Santa Barbara Basin, Ranges and western Transverse Ranges is missing in the Santa Maria Basin. Ventura Basin, San Joaquin Basin, and other subbasins (Atwater and Stock, Instead, Miocene sedimentary rocks are deposited nonconformably on the 1998; Hornafius, 1985; Luyendyk, 1991; Crouch and Suppe, 1993; Nicholson Franciscan Formation and record the development of a deep Miocene basin et al., 1994). The western Transverse Ranges began to rotate clockwise at (Woodring and Bramlette, 1950). At the base of the Miocene section, there is 18 Ma, and transtensional basins developed at the boundaries of the rotating the nonmarine Lospe Formation, which is overlain by the shallow-marine Point domain to accommodate differential movement between the rotating crust Sal Formation, and then the deeper-marine Monterey and Sisquoc Formations. and the nonrotating crust to the north and south (Luyendyk, 1991; Crouch and The basin reached its maximum depth during deposition of the Monterey For- Suppe, 1993). The Santa Maria Basin developed along the northern bound- mation, which consists of biogenic shale and chert and is the primary source ary of the rotating western Transverse Ranges. Miocene transtension was of petroleum in the region (Woodring and Bramlette, 1950). The Sisquoc For- replaced by transpression along the plate boundary due to changes in Pacific mation transitions from deep-water shale to shallow-water sandstones and plate motion during Pliocene time. This change resulted in a component of records the beginning of shallowing of the basin. Pliocene sedimentary units shortening across the plate boundary that lifted the California Coast Ranges include the Foxen Mudstone overlain by the Carreaga Sand, both of which

Santa Maria River KEY 120.6° W +_ 120.1° GR 91 6 pIR-IRSL dates (ka) 97 +_ 6 Pezzoni Fault Santa Maria Orcutt Formation Line 1 Orcutt base points 34.9° 34.9° N Line 2 Line 4 Subsurface data +_ Point Sal NSC 98 5 (water or oil/gas wells) Line 3 Quaternary faults and folds Casmalia Hlls Fault Zone (dashed where blind) Lions Head Fault 1.8 - 2.5 GR 91 +_ 6 -Reverse/Thrust +_ (teeth on hanging wall) 97 6 DR 110 +_ 17 -Strike-Slip 99 +_ 6 (arrows show sense of motion)

1.7 - 2.4 Line 5 -anticline axes Hosgri Fault Zone 1.0 - 2.3 LR 110 +_ 7 Line 6 119 +_ 8 Casmalia Hills ? 1.7 - 2.5 rock uplift rates (mm/yr) Foxen Cyn. Fault San Rafael Mtns 0.9 - 1.4 underlying detachment slip rate= 5.6 - 6.7 mm/yr 1.7 - 2.6 Los Alamos

Vandenberg Los Alamos Fault Terrace Purisima Hills Little Pine Fault 3.3 - 4.9 Santa Ynez River RR 85 +_ 6 93 +_ 6

Santa Ynez River Fault Baseline Fault Santa Ynez River Fault Solvang ? Honda Fault Elevation 800 m 34.57° 34.57° 10 km 600 m 400 m Point Arguello 200 m Santa Ynez Fault 120.6° N 120.1° 0 m

Figure 2. Map of the Santa Maria Basin showing faults, anticline axes, the extent of the mapped Orcutt Formation at the surface, the Orcutt basal surface control points, cross sections, post-infrared–infrared-stimulated luminescence (pIR-IRSL) sample sites, and rock uplift rates mapped on a 5-m-resolution digital elevation model.

onlap older units in some places or are missing altogether, indicating local Coast Ranges of California (Page et al., 1998). In the Santa Maria Basin, the uplift along folds that started to grow up through the shallow sea in early Paso Robles Formation is deposited conformably on the underlying Carreaga Pliocene time (Behl and Ingle, 1998; Woodring and Bramlette, 1950). Deposi- Sand, indicating that little deformation was occurring within the basin during tion in the Santa Maria Basin was entirely subaerial by Pleistocene time, as the initial stages of deposition, despite the uplift of the surrounding ranges recorded by the terrestrial Paso Robles Formation. The Paso Robles Formation that were the source of the Paso Robles clasts. Intense deformation both within consists primarily of coarse conglomerates with clasts derived from the San and at the margins of the Santa Maria Basin followed deposition of the Paso Rafael and Santa Ynez Mountains (Dibblee and Ehrenspeck, 1989). This unit Robles Formation, and the unit is folded along west- and northwest-striking is present across the entire basin, as well as throughout the southern Coast axes and offset by the major faults that bound the edges of the basin (Dibblee Ranges, and has been interpreted to record the onset of uplift of the modern and Ehrenspeck, 1989).

The Orcutt Formation is a late Quaternary unit that overlies the Paso Robles This shows that most of the deformation of the Pleistocene Paso Robles Formation in the Santa Maria Basin, and it was the focus of this study, so we Formation, and older units, occurred prior to Orcutt deposition. The basal describe it in greater detail than the older units. The Orcutt Formation has contact of the Orcutt Formation is also deformed and has been tilted (as been mapped over an extensive area in the western portion of the Santa Maria much as 20°) along the flanks of the topographic highs and lifted to the top Basin between the San Rafael and Santa Ynez Mountains (Fig. 2; Woodring and of the Casmalia and Purisima Hills. The Orcutt Formation was initially depos- Bramlette, 1950; Dibblee and Ehrenspeck, 1989). The Orcutt Formation was first ited on a very low-relief surface (Woodring and Bramlette, 1950; McGregor, described by Woodring and Bramlette (1950), who interpreted the formation as 2019) that eroded previous topography associated with deformation of the an extensive fluvial terrace deposit that originally extended from the foot of the Paso Robles Formation. The fact that the Orcutt basal contact was initially a San Rafael and Santa Ynez Mountains to the ocean. They noted that the ero- peneplain sloping to sea level that was later deformed across the modern sional surface upon which the Orcutt Formation was deposited merges with a topography makes it an excellent marker for late Quaternary deformation shoreline platform at the coast. The formation consists primarily of poorly sorted and topographic development. sand with pebbles and stringers of gravel, and it has a maximum thickness of ~30 m (Woodring and Bramlette, 1950). A detailed stratigraphic description of the unit is beyond the scope of this study, but in the outcrops we observed, there Structures was typically interbedded gravel and sand in the lower 5 m of the unit, with an increasing amount of sand and silt upward (see Fig. 3; stratigraphic columns The triangular-shaped Santa Maria Basin is structurally bound to the north- in the Appendix). Within a few kilometers of the coast, the deposit consists of east by the northwest-striking Little Pine–Foxen Canyon fault zone, to the south well-sorted medium to coarse sand with a basal layer of shell hash and cobbles by the east-striking Santa Ynez River fault zone, and to the west by the offshore derived from the underlying Monterey or Sisquoc Formations. These cobbles, north-striking Hosgri fault zone (Fig. 1). Within the Santa Maria Basin, west- and the surface of the Miocene formations at the contact, contain pholad bor- and northwest-trending folds and faults show evidence of late Quaternary ings that indicate a nearshore environment and confirm the interpretations by activity, demonstrated by the tilted basal contact of the Orcutt Formation and Woodring and Bramlette (1950) that the basal erosional surface of the Orcutt faulted fluvial terraces (Woodring and Bramlette, 1950; Clark, 1990; Guptill et Formation merges with a shoreline platform at the coast. al., 1981; Tyler, 2013). The major structures within the basin are described and The Orcutt basal contact is an angular unconformity in most places, and geographically categorized here into two uplifted fold trends that result in topo- the Orcutt Formation overlies folded Miocene through Pleistocene rocks. graphic highs in the west-central Santa Maria Basin—the Casmalia Hills and

North South

sand/silt Figure 3. Photograph showing an outcrop of the Orcutt Formation (Lompoc Road site). The basal contact of the Or- cutt Formation at this location is ~4 m lower than the lowest exposure shown sand in photograph.

2 meters sand sand/silt

channelized gravel

the Purisima Hills (Fig. 2). Both these topographic features are anticlinoria that northern edge of the eastern Purisima Hills and extends another 30 km to the deform Miocene through Pleistocene sedimentary rocks. The Casmalia Hills eastern end of the Santa Maria Basin (Fig. 2). An asymmetric syncline with a fault zone is located along the north slope of the Casmalia Hills and includes steep south limb and less steep north limb underlies the Los Alamos valley the Pezzoni, Casmalia, and Orcutt frontal faults (Fig. 2). These are south-dipping between the Casmalia and Purisima Hills. reverse faults with similar orientations and estimates of offset and have been described as a continuous fault zone (Sylvester and Darrow, 1979; Gray, 1980; Clark, 1990; Namson and Davis, 1990). Previously published geologic cross ■■ METHODS sections across the Casmalia Hills interpreted a north-vergent, asymmetric fold overturning Miocene to late Pliocene units above the Casmalia Hills fault zone, Luminescence Dating suggestive of fault-propagation folding (Clark, 1990; Namson and Davis, 1990). The Casmalia Hills fault zone merges to the east with the Baseline–Los Alamos We used luminescence techniques to date the Orcutt Formation. Lumines- fault, which is a south-dipping reverse fault that offsets Quaternary deposits cence dating utilizes quartz or feldspar grains in sedimentary deposits younger (Sylvester and Darrow, 1979; Guptill et al., 1981; Tyler, 2013). than ca. 300 ka to determine the last time the sediment was exposed to light A single anticline is present in the western Purisima Hills that splits into (Aitken, 1998). We collected samples from five sites in the Orcutt Formation several parallel anticlines and synclines in the eastern Purisima Hills (Fig. 2). (Fig. 2). We took a “lower” sample at the base and an “upper” sample near the The Lions Head fault is interpreted to underlie the western Purisima Hills, but top of the exposure for each of the five sample localities (Table 1; Appendix). it is only exposed near the coast on the south side of Point Sal, where it has Samples were collected by removing the outer 40–50 cm of outcrop, driving been described as a steeply northeast-dipping oblique-slip fault (Gray, 1980; a metal pipe (capped on the outer end) into the exposure wall, excavating Clark, 1990; Gutiérrez-Alonso and Gross, 1997). Sylvester and Darrow (1979) around the pipe, and then capping the inner end while under a tarp to prevent interpreted a blind Lions Head–Purisima Hills fault to merge with the Base- light contamination. For the North Slope Casmalia site (NSC; Fig. 2; Table 1), line–Los Alamos fault to the east. The Baseline–Los Alamos fault marks the we used only the lower sample collected because the upper sample returned

TABLE 1. POST‑INFRARED–INFRARED‑STIMLATED LMINESCENCE pIR‑IRSL SAMPLE DATA Sample Location Th Dose rate Euivalent No. of Age ppm ppm Gy/k.y. dose aliuots ka Lat Long s N W NSC: North Slope Casmalia Lower 34.933333 120.5422222 2.6 2.1 7.09 4.9 to 5.31 5745 136 24 9 5

RR: Rucker Road pper 34.67555556 120.4391667 2.3 2.71 .27 4.5 to 5.12 476 13 23 5 6

RR: Rucker Road Lower 34.67611111 120.4394444 2.13 3.01 10.43 4.96 to 5.15 5361 233 10 93 6

LR: Lompoc Road pper 34.202777 120.525 2.24 1.15 5.24 5973 12 16 3.88 142 ± 9

LR: Lompoc Road Lower 34.202777 120.525277 2.62 2.15 6.2 4.56 to 4.95 6400 16 12 119

GR: Graciosa Road pper 34.5639 120.449 2.63 1.67 6.55 4.4 to 4.7 4724 115 24 91 6

GR: Graciosa Road Lower 34.5666667 120.449 2.6 1.1 6.3 4.3 to 4.77 5010 99 24 97 6

DR: Dominion Road pper 34.4305556 120.33639 2.33 1.165 4.29 504 132 16 3.88 119 ± 8

DR: Dominion Road Lower 34.4305556 120.3330556 2.61 1.59 6. 4.2 to 4.6 5024 95 16 99 6 Note: pper and lower refer to relative stratigraphic position of samples at a site. Italics indicate dose rates that were considered anomalous and not used, and the ages that would have resulted from these dose rates see tet for eplanation. Bold tet indicates preferred dose rate and age for two samples. Ranges in dose rates reflect possible range in moisture content history for each sample.

a late Holocene age, implying that we erroneously sampled younger material well data from Worts (1951) to generate points for the Orcutt basal contact on top of the Orcutt Formation at that site. where it is buried in the Santa Maria Valley. We used the post-infrared–infrared stimulated luminescence (pIR-IRSL) We added interpolated data points to the grid where the Orcutt Formation protocol from Rhodes (2011) for measuring the infrared luminescence signal has been eroded off the crests of the Casmalia and Purisima Hills anticlinoria. of multiple grains and estimating the age of burial. Previous studies in south- These interpolated points were graphically determined by first calculating dip ern California (Rhodes, 2011) and work on terrace chronology in the Santa from three-point problems of the Orcutt Formation’s basal contact on either Maria Basin by Tyler (2013) and Farris (2017) showed that IRSL gives results side of an anticline and then linearly projecting the basal contact orientations more consistent with soil development and expected incision rates, whereas from both the north and south limbs of the folds to their intersection points. optically stimulated luminescence (OSL; blue light) on quartz grains typically These points were always above the modern topography, suggesting that the results in unreasonably young ages (e.g., Pytlewski, 2020). The pIR-IRSL pro- pre-erosion Orcutt surface was higher than the modern crests of the Casmalia tocol of Rhodes (2011) eliminates the anomalous fading problem that often and Purisima Hills. We assume the folded Orcutt surface would have flattened affects normal IRSL measurements. closer to the anticline axes and was lower than the intersection point of our We used multigrain single-aliquot regenerative dose protocol (SAR) to two straight-line projections. However, since we do not know the exact shape determine the equivalent dose for each sample in the luminescence laboratory of the eroded parts of the folds and hence the exact original elevations of the at California State University, Long Beach, California. Measurements of the Orcutt surface at the fold axes, we used the elevation of the highest topography radioactive contents of the sampled material were made at the U.S. Geological as the elevation for our interpolated points. This assumes that there has been Survey (USGS) Luminescence Geochronology laboratory in Denver, Colorado. minimal erosion of the crests of the hills below the original Orcutt contact. Thus, We then used these measurements along with the sample depth, altitude, and the elevation of these interpolated points are most likely minimum estimates. full range of possible moisture content history to calculate a range in dose We gridded the point array into three separate surface tiles—west, central, rates for each sample. Probability density functions (PDFs) and radial plots and east—for maximum conformance of the interpolated surface to the data. were used to display aliquot statistics (Appendix), and equivalent dose values This increased the spatial distribution of data points per grid and minimized were calculated using a central age model (Galbraith and Roberts, 2012; Kars the influence of regional, west-sloping topography on calculated elevations. et al., 2012). We determined the full range of possible ages for each sample We applied a tension spline interpolation method in ESRI ArcGIS 10.4 to by dividing the range in dose rates by the range in possible equivalent dose this three-dimensional point array. Choosing an interpolation method in a values. Table 1 shows the average age and error derived from the full range geographic information system (GIS) that produces a surface of maximum con- of possible equivalent dose values. formance to the data points and is accurate in depicting the mapped geology was done through iterative constructions with both quantitative and qualitative evaluation. Qualitative evaluation was done using three-dimensional (3-D) Orcutt Formation Basal Contact Reconstruction visualizations of the surface (Fig. 4) and cross sections through the surface (Figs. 5 and 6) to view locations where the Orcutt basal surface intersected the To document Quaternary deformation across the Casmalia and Purisima topography and confirm that these relationships were consistent with map anticlinoria, we used the unconformity at the base of the Orcutt Formation as patterns of the Orcutt Formation’s basal contact. a marker horizon. We constructed a three-dimensional surface representing the base of the Orcutt Formation using basal contact geometries from our own observations and existing maps (Dibblee and Ehrenspeck, 1989) along Structural Analysis: Modeling Folding and Fault Slip with a 4-m-resolution interferometric synthetic aperture radar (IfSAR) digital elevation model (2.2 m vertical accuracy, 4.3 m horizontal accuracy) from We used area-balanced forward modeling to estimate the amount of fault the National Oceanic and Atmospheric Administration’s digital topographic/ slip required to produce the observed two-dimensional fold shapes. Fault mod- bathymetric database (Fig. 4). The points used in gridding three-dimensional els aim to reproduce geometric attributes of structures such as horizon dips surfaces in Arcmap 10.4 were plotted by first georeferencing 7.5 min geologic and the distribution and angle of unconformities across a section in order to quadrangles (1:24,000 scale) from Dibblee and Ehrenspeck (1989) to the IfSAR describe structural development and calculate fault slip. We created six two-di- digital elevation models. Places where the Orcutt Formation contacted an older mensional (2-D) cross sections from the interpolated 3-D surface representing unit (i.e., its basal contact) were then manually digitized with points along the the base of the Orcutt Formation. The orientations of these cross sections were contacts. We determined latitude, longitude, and elevation for each point from perpendicular to the major anticlinal axes (Fig. 2). Our cross sections do not the georeferenced IfSAR DEM. Error in the digital elevation model is a low account for any lateral component of slip that may be accommodated by the percentage of the uplift and fault slip measurements and therefore not con- faults. Structural relief (the difference between the maximum and minimum ele- sidered a significant source of uncertainty. We also incorporated groundwater vations of the Orcutt basal surface) was measured across the cross-section lines.

View looking southeast eastern Purisima Hills

western Purisima HIlls

Casmalia Hills 29 Vandenberg Terrace 0

2 3 0

60 Santa Maria Valley

1 0 3 5 0

Orcutt basal contact surface

N 305 5 km

- 15 Point Sal

Santa Ynez Mountains San Rafael Mountains eastern Purisima Hills

western Purisima HIlls

Casmalia Hills Vandenberg Terrace

Santa Maria Valley

Orcutt basal contact surface on topography

5 km

Point Sal N

Figure 4. (A) Oblique view looking southeast at the Orcutt basal surface from a point above Point Sal. (B) Same view of the Orcutt basal surface but with a three- dimensional (3-D) representation of the topography to illustrate the similarity between the surface and the present-day topography. Contours are shown in 15 m intervals.

Casmalia Hills topographic profile SW Line 1 NE 300 m dip of underlying units ? 150 m ? ? SL 315 meters m.s.r Orcutt Formation basal contact 2.5 X vertical exaggeration topographic profile Lions Head F. Casmalia F.Z. interpolation data points

17,374 m

Casmalia Hills SW NE Line 2 topographic profile 300 m Vandenberg Terrace 150 m SL 295 meters m.s.r dip of underlying units 2.5 X vertical exaggeration Casmalia F.Z.

27,523 m

Purisima Hills Casmalia Hills SW Line 3 topographic profile NE 300 m

150 m 239 meters m.s.r

2.5 X vertical exaggeration Casmalia F.Z.

18,347 m

Figure 5. Cross-section profiles (lines 1 through 3) of the basal Orcutt surface across the Casmalia and Purisima Hills. Red lines are the Orcutt Formation basal contact, and black lines are the topographic profile. Dip indicators of the underlying geologic units are shown where they were available on geologic maps. Maximum structural relief (m.s.r) is labeled and is defined as the vertical distance between the highest and lowest points of the Orcutt surface. Interpolation data points are displayed along line 2 to show conformance of the surface to the basal contact point array derived from borehole data and geologic mapping. F.—fault; F.Z.—fault zone; SL—sea level.

We used StructureSolver software (Eichelberger et al., 2015) to restore and displacement history and fault propagation assuming a constant propaga- forward model the cross sections. Forward modeling utilizes 2-D kinematic tion-to-slip ratio. This enabled us to estimate the trishear parameters, fault models of fault-related folding to compute fold shapes above modeled faults. geometries, and amounts of displacement along a fault that provided the best Our area-balanced forward models used symmetric trishear (Erslev, 1991; visual fit to the data. A systematic approach was applied by first evaluating Zehnder and Allmendinger, 2000) to approximate fault-propagation folding previous interpretations of fault geometries and depths (Namson and Davis, at the fault tip, with inclined shear above fault bends (Xiao and Suppe, 1992) 1990; Seeber and Sorlien, 2000; Shaw and Plesch, 2012) and modifying model in order to predict horizon shapes in the forelimbs and backlimbs of folds. parameters to reproduce our assemblage of data. Unique structural solutions Trishear is a preferred method of modeling propagation folding because this arose through an iterative process of assessing all structural solutions while method can reproduce the tightening of folds and the progressive tilt of strata prioritizing geometric constraints in the data. We chose a model that best with depth, asymmetric folding, and the rotation of bedding toward the fault, fit the collective data and honored the key constraints, which included well all of which are characteristics of structures in the Santa Maria Basin. Vari- data, the Orcutt basal surface, geologic contacts (controlling the fold shape ables that were manipulated in the fault models included the shape of the of deeper Miocene to Pliocene horizons), and bedding dips. Our model error faults; trishear parameters such as initial and final fault tip depths and the (fit of the final models to the surface and subsurface data) was 300 m or less triangular shear zone area (half-apical, trishear angle); the axial shear angle and was mostly due to a mismatch of Miocene horizons to well data that is at fault bends; and regional horizon depths for the hanging-wall and foot- discussed below. Modeled fault displacement error for the base Orcutt surface wall blocks. These variables allowed for modeling of pregrowth and growth was ±10 m, which is the range of variation allowed before the model began stratigraphy that could incorporate simple erosion in order to specify fault to violate key data constraints.

Casmalia Hills SW Purisima Hills NE Line 4 topographic profile 365 m 244 m 122 m SL 316 meters m.s.r - 122 m dip of underlying units Casmalia F.Z.

24,542 m

Purisima Hills NE SW Gato Ridge anticline Line 5 topographic profile 365 m 244 m

122 m + SL - 122 m 401 meters m.s.r

Los Alamos F. Foxen Cyn F.

25,755 m

Purisima Hills SW NE

Line 6 topographic profile Gato Ridge anticline 365 m + 244 m 122 m 323 meters m.s.r SL - 122 m Foxen Cyn F. Los Alamos F.

24,140 m

Figure 6. Cross-section profiles (lines 4 through 6) of the basal Orcutt surface across the Casmalia and Purisima Hills. Red lines are the Orcutt Formation basal contact, and black lines are the topographic profile. Dip indicators of the underlying geologic units are shown where they were available on geologic maps. Maximum structural relief (m.s.r) is labeled and is defined as the vertical distance between the highest and lowest points of the Orcutt surface. Refer to Figure 5 for key. F.—fault; F.Z.—fault zone; Cyn—Canyon; SL—sea level.

■■ RESULTS Orcutt Formation at this site is an expansive terrace that extends ~15 km west to the coastline (Vandenberg Terrace on Fig. 2), and this surface probably rep- Luminescence Dating resents the original upper surface of the Orcutt Formation. This geomorphic feature and the young dates from this site suggest to us that the ca. 85 ka age We obtained pIR-IRSL ages from the Orcutt Formation that indicate an age likely represents the later stages of Orcutt deposition. of ca. 120 ka for its base and an age of ca. 85 ka for its top. The Lompoc Road For the majority of samples, aliquot data were tightly grouped, and there site (Fig. 2) returned the oldest ages with a lower sample age of 119 ± 8 ka was no evidence of partial bleaching or bioturbation in the frequency dis- (Table 1). This site was located at the base of the Orcutt Formation, and thus tribution histograms (Appendix). Dose rates were consistent between sites, this age most closely represents the start of Orcutt deposition. The youngest with most between 4.5 and 5 Gy/k.y. Two sites, however, had inverted ages, ages were from the Rucker Road site, where the upper sample returned an where the stratigraphically higher “upper” sample was 15–20 k.y. older than age of 85 ± 6 ka. This site was the thickest exposure (13 m) we sampled, and the “lower” sample, which is not possible (Lompoc Road Upper and Dominion the base of the formation was not exposed at the site. The top surface of the Road Upper; Table 1). An apparent age inversion could be caused by partial

bleaching of the upper samples or by inaccurate determination of dose rates. the southwest of Point Sal, the Orcutt basal surface flattens in the Vandenberg Neither of the two samples showed strong evidence of partial bleaching, which Terrace area and transitions into a shoreline platform along the coast. would normally be expressed as an asymmetric PDF plot of aliquot equiva- The six cross sections through the Orcutt surface provided reference lent doses (Appendix). We therefore believe that the age inversion was most points for rock uplift measurements and deformation modeling along tran- likely due to inaccurate determination of dose rates at these two sites. Dose sects through the Casmalia and Purisima Hills. Structural relief between the rates for the upper samples at both sites were anomalously low compared lowest and highest Orcutt basal surface points ranges from 238 m across to the other samples in this study, which results in an older age. This may be line 3 (eastern Casmalia Hills) to 400 m across line 5 (central Purisima Hills). due to an inaccurate measurement of the U, Th, or K concentrations in the dose rate sample or error in our estimation of water content over the lifetime of the deposit, all of which affect the final dose rate determination. Although Fault Displacements from Structural Forward Modeling we cannot confidently attribute the apparent age inversion of samples at the Lompoc Road and Dominion Road sites to a specific cause, we believe the dose Two cross sections (lines 2 and 4) were used to construct area-balanced rates for the upper samples at these locations are not representative and are solutions for fault geometries at depth, position of the main fault tip, regional not consistent with the lower samples, and we therefore adopted dose rate horizon depths, fault displacement, and the distribution of contractional strain values from the lower samples at each of these locations to get a more equal across each section (Figs. 7 and 8). The locations of these lines were cho- approximation of the ages of the upper and lower samples. sen where we had the greatest density of surface and subsurface data. The resultant models contain deformed horizon geometries that most accurately conform to independent geometric constraints from oil and gas wells, ground- Deformation of the Orcutt Basal Contact water wells, geologic maps, and 3-D structure contour data.

The geometry of the Orcutt basal surface (Figs. 4, 5, and 6) generally reflects the modern topography. The horizon is folded over the Purisima and Casmalia Cross-Section Line 2 Hills and shows a distinct northeast-vergent asymmetry, with steeper dips on the north side of the Casmalia Hills and eastern Purisima Hills (Figs. 5 and 6). Our preferred model for the regional structure responsible for folding of One area of uncertainty is the northwest end of the study area, where a lack the western Casmalia anticlinorium includes a southwest-dipping reverse of Orcutt deposits preserved in the hanging-wall block of Point Sal made it fault with the fault tip buried beneath 1.5 km of sediment (Fig. 7). Incline shear impossible for us to confidently reconstruct the Orcutt surface. Our contoured fault-bend-folding was used to model the backlimb shape and symmetric surface, based on control points on the edges of the elevated area, intersected trishear fault propagation folding (Erslev, 1991) with less than a kilometer of the high topography of Point Sal (Fig. 5, line 1), which suggests that this topo- propagation for the forelimb. graphic high existed during Orcutt deposition, and the Orcutt fluvial system Computed fault displacements to account for the deformation of stratigraphic flowed around it. This may not be accurate, however, because our Orcutt sur- horizons are shown on Figure 7 and include 731 m for the late Pleistocene base face interpolation does not account for Quaternary dip-slip displacement along Orcutt horizon. The uncertainty of these fault displacements is ±10 m, which the Pezzoni and Lions Head faults that bound the high topography of Point Sal comes from the variability in fault slip allowed by the model to best fit the data on the north and south sides. The Pezzoni fault is a south-dipping reverse fault constraints. We note that the model required greater fault displacement in the with ~3000 m of displacement of Miocene units across the fault (Woodring Pliocene Foxen mudstone than in the older and deeper Sisquoc and Monterey and Bramlette, 1950). Little is known about the recency of faulting, but the Formations (Fig. 7), which implies earlier normal slip along the fault. This agrees fault appears to truncate Orcutt deposits on the north side of Point Sal, and with previous interpretations, which inferred that many faults in the region were an unpublished consulting report (Woodward-Clyde Consultants, 1988) noted originally Miocene normal faults that were reactivated as reverse faults in Plio- apparent offset geomorphic surfaces that led them to infer late Pleistocene cene time (e.g., Namson and Davis, 1990; Crouch and Suppe, 1993; Sorlien et displacement. On the south, the Lions Head fault is a steeply north-dipping al., 1999). Average dip of the upper section of the fault is 17° to the southwest, reverse fault that offsets a late Quaternary marine terrace by up to 9 m along decreasing to 10° dip near a depth of 2743 m. Depths to the Orcutt, Paso Robles, the coast (Clark, 1993). We therefore infer that the Orcutt Formation was likely and Careaga stratigraphic horizons in Santa Maria Valley and the location and lifted above the current topography of Point Sal by reverse slip on the Pezzoni orientation of the distinct late Pleistocene angular unconformity on the northern and Lions Head faults and was subsequently eroded. We did not include this Casmalia Hills were reproduced in this model. The fold shape of the base Orcutt area in our measurements of rock uplift, however, because of the uncertain- horizon, depths to deeper horizons on the fold crest and forelimb, dip meter ties associated with post-Orcutt offset along these faults and whether the data, geologic contact locations on the forelimb and backlimb, and bedding Orcutt Formation abutted or was lifted over the older rocks at Point Sal. To dips across the structure were also honored by this model.

Cross-Section Line 4 relief and northeast tilting of overlying units in the northern Purisima Hills and controls the moderate south dip and thickness of Pliocene to Pleistocene units In our preferred model for line 4, the late Pleistocene and older horizons in the backlimb of the eastern Casmalia Hills anticlinorium. are folded above a north-dipping reverse fault and a deeper south-dipping, Shallow deformation and tight, asymmetric folding within the Purisima reverse detachment (Fig. 8A). This cross section was split into two models Hills anticlinorium could only be reproduced by a combination of trishear (Figs. 8B and 8C), and both models require fault propagation above the fault tip fault propagation and fault-bend-folding above a listric, north-dipping reverse to reproduce observed asymmetric folding. The north-dipping fault beneath the fault (Fig. 8B). Symmetric trishear was used to model propagation at the western part of the Purisima Hills anticlinorium soles into a midlevel detach- fault tip and was preferred over the south-dipping fault ramp model of Nam- ment within mid-Miocene deep-marine strata, while the south-dipping regional son and Davis (1990). We prefer the trishear model because the amount of fault continues to dip through the section at a low angle beneath the Purisima structural relief, strong south-vergent asymmetry of folded units, bedding Hills and the north-dipping fault. The moderate northeast dip of the late Meso- dips, and distribution of the geologic contacts among the Sisquoc Formation, zoic–early Cenozoic rocks below the midlevel, north-dipping fault is produced Foxen mudstone, and Careaga Formation exposed on the northern limb of by fault-bend-folding over a deep fault ramp (~5000 m depth) along the deeper, the anticline could not be reproduced from only fault-bend-folding above a south-dipping fault. Slip along the deeper fault also contributes to the structural south-dipping ramp. This model does not discount the existence of a fault

Forward modeling of structural cross-section Line 2 SW NE

Vandenberg Terrace Casmalia anticline bedding dips base-Orcutt surface eroded surfaces 32° half-apical angle topographic profile 500 m

? 731 SL ? Tca QTpr 3171 ? - 500 m 2929 Tf 4008 Tsq

ult 3890 t Fa rcut - O Tm - 2,000 m 3926 alia asm shear axis (80°) C model surface fault displacement (m) needed to produce shape of horizon ? ? 27.5 km -5000 m EXPLANATION (for Figs. 7 and 8)

Geologic Units Borehole ? Fault; dashed where approximate, queried where uncertain Qo Orcutt Fm (119-85 ka) Tm Monterey Fm. (mid to late Miocene) Lithostratigraphic Tpr Paso Robles Fm. (Plio-Pleistocene) Tps Point Sal Fm. (mid Miocene) horizon Spline Interpolated Orcutt surface Tca Careaga Fm. (late Pliocene) Tl Lospe Fm. (early Miocene) Eroded model surfaces Dip meter data Tf Foxen mudstone (early - mid Pliocene) Fr Franciscian/Knoxville (Jurassic?) Model surface Tsq Sisquoc Fm. (late Miocene) Undifferentiated sedimentary and basement rocks Geologic contact 3 km (Dibblee and Ehrenspeck, 1989) no vertical exaggeration

Figure 7. Area-balanced, fault-related fold model solution for the western Casmalia Hills. Shear axes, trishear parameters, and horizons depths were adjusted to fit the shown geometric constraints and produce fault displacements. The fault displacement arrows show the direction (fault parallel) and magnitude (m) of displacement along the fault that produces the above fold shape of a particular horizon in our preferred geometric models. For some horizons, there is a greater amount of displacement in younger units relative to older units beneath, which is the result of an earlier episode of normal slip along the fault. Note that geologic and subsurface data indicate very steep, and in some places overturned, bedding dips on the north side of the Casmalia Hills anticline, showing that a significant amount of folding occurred prior to planation and deposition of the Orcutt Formation. SL—sea level.

Forward modeling of structural cross-section Line 4 SW NE Purisima anticline bedding dips Casmalia anticline ' eroded surface base-Orcutt surface 500 m' Qo SL Tf Tpr Tca

Tf -1500 m

Tsq ?

- 3000 m Tm Tps Tl lt au tt F cu Fr Or model surface ia - mal Cas

basement

? 3 km ? no vertical exaggeration

27 km - 7000' m LINE 4 - south LINE 4 - north

35.7° NE SW 57.8° half-apical angle topographic pro le 500 m 481 SL 249 SL 1060 1262 1162 1737 1305 3363 1292 -1500 m 3108 1096 830 4432 shear axis (85°) ? 4192 - 3000 m 3750 3592 3 km 3592 shear axis (85°)

no vertical exaggeration model surface 27 km fault displacement (m) needed to produce folded shape for each individual horizon - 7000 m

Figure 8. (A) Combined fault-related fold model showing the spatial relationship between the south-dipping deep detachment and north-dipping reverse fault across line 4. (B, C) Area-balanced forward model solutions that constitute the composite with: model attributes, fault displacements of stratigraphic horizons, and geometric data used in the modeling. The fault displacement arrows show the direction (fault parallel) and magnitude (m) of displacement along the fault that produces the above fold shape of a particular horizon in our preferred geometric models. Note that geologic and subsurface data indicate very steep, and in some places overturned, bedding dips, showing that a significant amount of folding occurred prior to planation and deposition of the Orcutt Formation. For the Miocene units, there is a greater amount of displacement in younger units relative to older units beneath, which is the result of an earlier episode of normal slip along the faults. Refer to explanation on Figure 7 for units and symbols. SL—sea level.

ramp along a deeper south-dipping detachment (which may contribute to the of the hills (Fig. 2). Two different methods were used, and the results for each northward dip of basement rocks below this fault), but a fault in the upper method are summarized in Table 2. section is still needed to best match the data. Other model iterations with In the first method, we measured incision of the Orcutt basal strath surface different geometry or dip directions for this fault did not accurately conform to a local base level represented by the elevation of the nearest active river to the collective data. For example, decreasing the detachment depth of our channels (along profiles parallel to the coast from the uplift measurement north-dipping fault model increased the fault planarity and angle between points). We inferred that the incision of these rivers, the Santa Maria, Los the fault and the shear axis. This resulted in a decrease in concavity of fold- Alamos, and Santa Ynez rivers, has kept pace with uplift because they have ing across the axis, more vertical displacement of horizons than folding, and equilibrium channel profiles that are graded to sea level (Kelty, 2020), and incongruence with Pliocene to Pleistocene stratigraphic contacts and dip in therefore the incision amount is a proxy for rock uplift amount (Pazzaglia et the backlimb (north side) of the fold. Our preferred model also honored the al., 1998). Because the Orcutt Formation in the western Santa Maria Basin magnitude and dip direction of dip meter data and depth to horizons deter- was deposited on a regional erosional surface that has been incised by mul- mined from lithologic logs (Fig. 8). tiple rivers, we cannot attribute planation of the basal surface at one of our The computed fault displacements across the north-dipping fault needed measurement points to a specific active channel in the modern topography. to account for the deformation of stratigraphic horizons in cross-section line Therefore, maximum and minimum rock uplift amounts reported in Table 2 4 are shown on Figure 8B and include 249 m for the base Orcutt horizon. The correspond to differences in the present-day elevation of the modern channels depth of the main fault tip (~1500 m) and <1 km of fault tip propagation were on either side of the Casmalia and Purisima Hills and represent our uncertainty a direct result of reproducing the data. The average dip of the upper section of in uplift (incision) amount. the fault is 24° to the north, and the fault decreases dip to a near-flat detach- For the second method, we measured rock uplift relative to sea level by ment at ~3000 m depth. comparing the lifted Orcutt basal surface on top of the Purisima and Casmalia To the north, the eastern Casmalia Hills anticlinorium is deforming above Hills to an inferred projection of its original slope to sea level. As mentioned a southwest-dipping reverse fault with a deep ramp contributing to north- previously, the fluvial Orcutt Formation grades into beach deposits near the east regional tilt in the backlimb. Fault propagation caused the north-vergent coast, and the Orcutt basal surface that was beveled by fluvial processes asymmetric folding above the fault tip, and the broad and flat crestal geometry inland becomes an elevated shoreline platform (marine terrace) along the results from small inflections in the fault geometry at depth. The upper section coastal sections of the Vandenberg Terrace (Figs. 2 and 9). This unique situa- of the fault dips ~34° to the southwest (Fig. 8) and decreases to a 10° dip at a tion allowed us to tie the Orcutt basal surface directly to sea level at the time depth of 4900 m. This model was able to reproduce the distinct late Pleistocene of its formation. The precise location of the paleoshoreline is not exposed, but angular unconformity between the Orcutt and Paso Robles Formations on the exposures of the Orcutt basal surface in canyons along the north side of the north slope of the Casmalia Hills, as well as obey the folded base Orcutt horizon Santa Ynez River show that the shallow-marine deposits transition to fluvial geometry, depths to the older horizons, geologic contacts, and bedding dips deposits somewhere between 5 and 6 km inland from the coast. The gradient across the structure. The location of the fault beneath the eastern Casmalia of the Vandenberg Terrace varies from 6 m/km (0.006) along a line parallel Hills anticlinorium matches the depth of a major fault observed in the Union to the Santa Ynez River to 10 m/km (0.01) along a straight line from the coast Dome 18 well (Fig. 8C). Fault displacements needed to create the observed to the western nose of the Purisima Hills. These slopes are steeper than the deformation related to this fault are shown in Figure 8C and include 481 m modern rivers (0.002–0.004) but shallower than the gradients measured on for the base Orcutt horizon. Like the displacements modeled in cross-section paleoshore platforms along coastal California (0.02–0.04; Bradley and Griggs, line 2, this model produces less slip in older units in the Miocene section, which 1976). The gradient of the Vandenberg Terrace is more consistent with a sur- reflects earlier normal slip on this fault during Miocene time. face formed by fluvial processes that graded to sea level, but some westward tilting has most likely occurred since its formation. To calculate rock uplift amounts, we projected the Orcutt basal contact ■■ DISCUSSION on the Vandenberg Terrace inland and compared this projected reference surface to the present-day elevation of the Orcutt basal surface on top of the Quaternary Rock Uplift Amounts and Rates Casmalia and Purisima Hills (Fig. 9). To account for the possibility of regional tilting of the Vandenberg Terrace, we calculated maximum and minimum uplift The correlation between the modern topography and Orcutt basal sur- amounts based on inferred original slope angles for the Orcutt basal contact face shows that the topographic evolution of the Casmalia and Purisima Hills projection in this area (Table 2). Our minimum slope was the minimum of the has occurred since deposition of the Orcutt Formation (Figs. 4, 5, and 6). We present-day stream gradients (0.002), and our maximum was the present-day therefore used the Orcutt basal strath surface as a marker to measure rock slope of the Vandenberg Terrace perpendicular to the coast and parallel to the uplift across the Casmalia and Purisima Hills at six locations along the crests Santa Ynez River (0.006), where we believe it is least likely to be deformed or

TABLE 2. PLIFT AMONTS AND RATES Reference point Present Elevation of Elevation above Rock uplift at coast Total rock uplift Age of Orcutt base plift rate elevation reference reference surface m elevation above ka mm/yr m m m paleo–sea level or active channel m Ma Min Ma Min Ma Min Ma Min Ma Min Ma Min

Relative to modern rivers Casmalia crest line 1 300 32 26 274 26 Not applicable 274 26 127 111 2.5 2.1 Casmalia crest line 2 25 66 55 230 219 for modern river 230 219 127 111 2.1 1.7 Casmalia crest line 4 290 166 79 211 124 channels assuming 211 124 127 111 1.9 1 Purisima crest line 2 160 4 9 151 112 incision of modern 151 112 127 111 1.4 0.9 channels has kept Purisima crest line 4 305 3 19 26 222 26 222 127 111 2.6 1.7 pace with uplift Purisima crest line 6 600 14 73 527 416 527 416 127 111 4.7 3.3 Relative to proected marine terrace/peneplain surface Casmalia crest line 1 300 12 96 204 172 7 56 22 22 127 111 2.5 1. Casmalia crest line 2 25 131 97 1 154 7 56 266 210 127 111 2.4 1.7 Casmalia crest line 4 290 190 130 160 100 9 76 25 176 127 111 2.3 1.4 Purisima crest line 2 160 92 74 6 6 63 41 149 109 127 111 1.3 0.9 Purisima crest line 4 305 190 150 155 115 12 106 23 221 127 111 2.5 1.7 Purisima crest line 6 600 272 172 42 32 120 9 54 426 127 111 4.9 3.4 Reference is modern active river channels for first group and the proected shoreline platform/fluvial strath surface for second group. To correct for uplift of reference surface at the coast and the difference in sea level between 119 ka and present 12.9 11 m from present; Simms et al., 2016. We used the age of the lower Orcutt Formation sample collected ust above the base of the formation 119 ka as an estimate of the beginning of Orcutt deposition. The elevation of the paleoshoreline at the coast and associated rock uplift assume the Lions Head fault has not been active since Orcutt time, which is based on geologic maps that show no offset of the Orcutt Formation by the fault.

Rock uplift calculation from projection of Orcutt shoreline platform / fluvial strath surface West 12x vertical exaggeration East 300 m rock uplift = A + B topo profile A deformed Orcutt fluvial deposits 200 m basal surface projected undeformed shallow marine/ shoreline deposits Orcutt basal surface 100 m elevation of Orcutt paleo-shoreline B shoreline platform at coast becomes Orcutt basalsea levelsurface at 119inland ka (+13 ± 11 m relative to modern sea level) 0 m modern sea level 2.5 km 5 km 7.5 km 10 km 12.5 km 15 km 17.5 km

Figure 9. Schematic cross section showing how rock uplift measurements were made relative to sea level. Inland projection of the Orcutt basal contact on the Vandenberg Terrace surface (green dashed line) is compared to the present-day elevation of the Orcutt basal contact (orange line) at a point on top of the lifted topography. This difference (A) is added to the uplift of the Orcutt basal surface under the Vandenberg Terrace since the development of the surface at 119 ± 8 ka (B).

tilted since it formed. Because the Vandenberg Terrace is a shoreline platform scope of this study, but it is likely related to deeper crustal thickening, possibly at its coastal extent, we added the uplift of the inferred paleoshoreline since along the detachment at 12–15 km depth that has been interpreted by multiple formation of the shoreline platform to the elevation of the Orcutt base at our studies (Levy et al., 2019; Huang et al., 1996; Namson and Davis, 1990). Our calculation points above the reference surface to get a total rock uplift value. rock uplift rates along the crests of the Casmalia Hills and Purisima Hills are We approximated the elevation of the paleoshoreline by estimating the ele- likely superposed on this regional uplift, such that the rate of rock uplift due vation of the Orcutt basal surface (3–5 m below the surface elevation) at the to the underlying blind faults modeled in this study are anywhere from near inferred paleoshoreline 5.5 km inland from the coast. 0 mm/yr at the western nose of the Purisima Hills to a little less than 4 mm/yr We believe an age of 119 ± 8 ka to be the best approximation of the Orcutt in the eastern Purisima Hills. basal surface because this date came from a sample collected less than 2 m above the Orcutt basal contact, and it was the oldest Orcutt age we measured in this study (Lompoc Road locality in Table 1). We therefore used this age to Late Pleistocene Fault Slip Rates calculate uplift rates and to determine paleo–sea level when the Orcutt basal surface was beveled. Sea level at 119 ka was +12.9 ± 11 m relative to modern A range of dip-slip rates for each fault was calculated by dividing the com- sea level based on a glacial-isostatic adjusted sea-level curve calculated for puted fault displacements from our kinematic models by the lower age (119 the area by Simms et al. (2016). ± 8 ka) of the Orcutt Formation that best approximates the age of the basal Our two separate methods resulted in similar rock uplift amounts and rates contact. For cross-section line 2, displacement along the fault is 731 ± 10 m, (Table 2), with neither method consistently producing faster rates than the which results in a dip-slip rate of 5.7–6.7 mm/yr. For cross-section line 4, dis- other. The rates plotted on Figure 2 represent the full range of maximum and placement along the north-dipping fault is 481 ± 10 m, and the dip-slip rate is minimum rates from both methods. Rock uplift rates average around 2 mm/ 3.7–4.4 mm/yr. Dip-slip displacement of 247 ± 10 m on the south-dipping fault yr along the crest of the Casmalia Hills and range from 1.0 to 2.5 mm/yr. The in line 4 results in a slip rate of 1.9–2.3 mm/yr (Fig. 8). rates are faster in the west, where the highest topography is present. Our rock The total shortening of 689–708 m across the fault in line 2 since 119 ± 8 ka uplift rates for the western Casmalia Hills are significantly higher than the uplift results in a shortening rate of 5.4–6.5 mm/yr. Cumulative shortening across rates of 0.14–0.18 mm/yr previously estimated by Clark (1993) near Point Sal. the two faults in line 4 is 4.9–6 mm/yr, i.e., slightly less than the rates calcu- For the Purisima Hills, uplift rates vary from 0.9 mm/yr in the west, where lated across line 2. These late Pleistocene shortening rates are comparable the topography is lowest, to 4.9 mm/yr in the east, where the topography is the to geodetic rates of northeast-directed (N30°E) shortening across the basin, highest. The relatively fast rates in the eastern Purisima Hills are unexpected, normal to the fold belt, which are 6 ± 2 mm/yr (Feigl et al., 1990). The similarity and we note that this location is the only measurement point where the Orcutt in rates suggests that the faults that underlie the Casmalia and Purisima Hills basal surface is not actually present at the top of the hills, but rather projected are accommodating the majority of shortening across the basin. Kelty (2020) based on our reconstruction of the Orcutt basal surface. It is possible that our has shown, however, that reverse slip on the Santa Ynez River fault occurred projection of the Orcutt basal surface over this area is incorrect and that some during this time period as well, and the unknown amount of shortening across high topography existed here during Orcutt time, and the Orcutt Formation that fault would need to be included in any estimate of the total north-north- was deposited as a buttress unconformity. The tilting of the Orcutt Formation east–oriented shortening across the basin since ca. 119 ka. along the northern and southern edge of the eastern Purisima Hills (as much The 2-D forward models in this study assumed pure reverse displacement as 20°), however, and the lack of any remnant Orcutt deposits in the higher on the faults and negligible lateral displacement. Most of the Casmalia Hills elevations suggest that our interpretation that the Orcutt surface projected fault zone and the faults that underlie the Purisima Hills do not reach the over the modern topography is correct and that there has been greater late surface, so there is no way to measure lateral slip directly at the surface. Pre- Quaternary uplift here than to the west. We interpret that the high topography vious studies, however, have inferred a component of left-lateral slip on these here is a result of the fast uplift rates, and the Orcutt Formation is not present faults, as well as the Lion’s Head and the Santa Ynez River faults. Sylvester and at the top of the Purisima Hills because it has been eroded off. Darrow (1979) interpreted the folds in the Casmalia Hills, Purisima Hills, and The slower rates in the western Purisima Hills occur where the anticlino- along the Santa Ynez River fault to be left-stepping en-echelon folds resulting rium flattens into the Vandenberg Terrace. We interpret the uplift rate of the from a component of left-lateral slip along these faults at some time in their Vandenberg Terrace to represent the regional uplift of the paleo-peneplain history. Clark (1993) and Sorlien et al. (1999) reported striations on fault sur- onto which the Orcutt Formation was deposited. This regional uplift rate is faces along the Lion’s Head and Honda faults that suggested a component of supported by research conducted for several master’s theses set in the Santa left-lateral slip. Conversely, comparison of crustal velocities west of the San Maria Basin that used fluvial or marine terraces to determine incision and Andreas fault in central California from geodetic data (Feigl et al., 1990; Shen uplift rates of ~1 mm/yr in the footwall blocks of the reverse faults (Kelty, 2020; et al., 2003; DeMets, 2014; Sandwell and Wessel, 2016) with the orientation of Farris, 2017; Tyler, 2013). The cause of this regional uplift rate is beyond the the faults we modeled suggests a component of right-lateral shear across the

faults. Focal mechanisms in the northwestern corner of the Santa Maria Basin Basin (Marshall et al., 2013; Hammond et al., 2017). Late Quaternary uplift (McLaren and Savage, 2001; Hardebeck, 2010) are mainly reverse sense, but rates determined from marine terraces near Ventura are as high as 7 mm/yr a few earthquakes include focal mechanisms that would indicate right-lateral (e.g., Rockwell et al., 2016) but range from 0.5 to 2 mm/yr farther west along displacement if they occurred on one of the main faults. Consequently, the the Santa Barbara coast (Gurrola et al., 2014; Morel, 2018). faults modeled in this study likely involve a component of lateral slip that we Deformation in the southern Coast Ranges, to the north of the Santa Maria are unable to detect. Any additional lateral slip would increase the total slip Basin, is characterized by right-lateral transpression and slower rates of defor- rate on these faults by some unknown amount and would mean the rates mation and uplift than in the western Transverse Ranges. Major faults in the presented here are minimum estimates. The contradiction between geologic southern Coast Ranges include the West Huasna, Rinconada, and Hosgri evidence for left-lateral slip versus geodetic and seismic evidence for right-lat- faults, which are steeply dipping, north-northwest–striking faults with pri- eral slip may be a result of a change in fault kinematics through time, with marily right-lateral strike-slip geologic offsets and focal mechanisms (e.g., earlier left-lateral slip replaced by later right-lateral slip. Page et al., 1998; Hardebeck, 2010). The Hosgri fault exhibits late Quaternary The true slip rates of these faults may also be larger than we calculated slip rates of 1–3 mm/yr at Point Buchon (Fig. 1; Hanson et al., 1992; Lettis et here because of our uncertainties in the timing of deformation. The slip rates al., 2004), and the West Huasna and Rinconada faults together accommodate we report are based on the oldest ages we obtained from the Orcutt Forma- up to 1–3 mm/yr of slip (Titus et al., 2007; Hardebeck, 2010). Uplift rates in tion, which assumes that deformation began during Orcutt deposition. The the southern Coast Ranges are also slower than in the western Transverse lack of unconformities or angular discordance within the Orcutt stratigraphy, Ranges. In the San Luis Obispo area (Fig. 1), late Quaternary marine terraces however, and the widespread deposition of this fluvial deposit, which is only show that individual ranges are being lifted at rates of 0.1–0.2 mm/yr along 30–50 m thick (Woodring and Bramlette, 1950), suggest that deformation and steeply dipping reverse faults with slip rates of 0.2–0.5 mm/yr (Hanson et al., uplift may not have started until after the Orcutt Formation was deposited. This 1992; Lettis and Hall, 1994; Lettis et al., 2005). These elevated areas have little is supported by the fact that fluvial terrace deposits in the Santa Maria Basin to no internal folding during Quaternary time, and deformation is concen- that are younger than the Orcutt Formation are confined to present-day drain- trated along the high-angle faults at the margins of the ranges. North of San ages, showing that topographic growth and localization of later fluvial deposits Luis Obispo, the Santa Lucia Range is being lifted at 0.8 mm/yr due to slip on occurred after the wide peneplain-like deposition of the Orcutt Formation. If steeply dipping reverse faults that bound the mountain range (Ducea et al., deformation and uplift did not start until the end of Orcutt deposition (ca. 85 ka), 2003). These reverse faults are likely the result of plate boundary transpression then the fault slip rates would be 8–9 mm/yr, and uplift rates along the crests and a restraining step geometry, where right-lateral displacement on faults of the Casmalia and Purisima Hills would increase by an average of 1 mm/yr. southeast of the range is transferred westward to the Hosgri fault (Clark et al., 1994; Titus et al., 2007; Johnson et al., 2014). The orientations of faults and folds in the Santa Maria Basin have a some- Implications for Regional Active Tectonics what northwestward-opening fanning geometry and vary from west-striking features, like in the western Transverse Ranges, to more northwest-striking The deformation style and rates in the Santa Maria Basin support previous features, like in the southern Coast Ranges. The style and rates of deformation interpretations that this area is a transition zone between the southern Coast along these structures, however, are more similar to the western Transverse Ranges and western Transverse Ranges that is accommodating differential Ranges. The low-angle blind thrust faults that cause folding at shallower levels motion between the two tectonic domains (e.g., Hornafius, 1985; Feigl et al., within the basin are similar to deformation patterns in the western Transverse 1990; Sorlien et al., 1999; McLaren and Savage, 2001; Lettis et al., 2004). High Ranges and are unlike the high-angle reverse faults and right-lateral faults rates of shortening and uplift have been documented in the western Trans- that characterize the southern Coast Ranges to the north (Namson and Davis, verse Ranges. Marshall et al. (2013) measured an average geodetic shortening 1990; Page et al., 1998; Sorlien et al., 1999; Wirtz, 2017). Focal mechanisms and rate of 7 mm/yr, while other geodetic studies have reported rates as high as limited geologic data indicate that right-lateral shear across the Santa Maria 12 mm/yr (Donnellan et al., 1993; Hagar et al., 1999). Measurements of geologic Basin area appears to be focused on the more northerly striking faults that mark shortening rates since Miocene time vary from 6.5 to 9.1 mm/yr (Levy et al., the margins of the basin, primarily the Hosgri fault to the west, but possibly 2019) to as high as 25 mm/yr (Yeats, 1983; Huftile and Yeats, 1995). Slip rates on the West Huasna and Foxen Canyon faults to the northeast as well (Sorlien faults in the western Transverse Ranges have mainly been determined in the et al., 1999; McLaren and Savage, 2001; Lettis et al., 2005; Hardebeck, 2010). Ventura area, where upper limits range from 7 to 11 mm/yr along the Ventura, The uplift rates and fault slip rates reported here are also similar to those San Cayetano, and Red Mountain faults (Fig. 1; Rockwell et al., 1988; Huftile seen in the western Transverse Ranges and are significantly faster than those and Yeats, 1995; Huftile and Yeats, 1996; Hubbard et al., 2014). Geodetic uplift in the southern Coast Ranges. Deformation rates in the Santa Maria Basin rates vary from 2 mm/yr in the northeastern western Transverse Ranges and decrease dramatically to the north across the Santa Maria River, where the along the San Andreas fault to −4 mm/yr in the actively subsiding Ventura northern part of the Los Osos domain of Lettis et al. (2005) near San Luis

Obispo (Fig. 1) is experiencing uplift rates and fault slip rates that are an order shortening has undoubtedly occurred across the Santa Ynez River fault to of magnitude slower than those in the Santa Maria Basin. The faster uplift the south (Wirtz, 2017; Kelty, 2020), and through layer-parallel shortening and rates may result from a greater degree of north-south shortening occurring outcrop-scale structures across the area (Gutierrez-Alonso and Gross, 1997; in the Santa Maria Basin as opposed to transpression in the southern Coast Wirtz, 2017). We doubt that this additional amount is anywhere near sufficient Ranges to the north. to accommodate 80 km of displacement on the Hosgri fault, and our data fit Our data fit previous models that attributed shortening and uplift across the better with interpretations of 10 km or less of displacement on the Hosgri fault Santa Maria Basin to ongoing vertical-axis rotation of the western Transverse at the latitude of the Santa Maria Basin (Colgan and Stanley, 2016; Sorlien et al., Ranges. Hornafius (1985) first interpreted the Santa Maria Basin to be shorten- 1999). The model proposed by Colgan and Stanley (2016) involved right-lateral ing as a result of rotation of the western Transverse Ranges. Lettis et al. (2005) slip along the faults within the Santa Maria Basin that transferred clockwise noted that a velocity gradient between the western Transverse Ranges and the rotation of the western Transverse Ranges into lateral slip on the southern southern Coast Ranges, which they observed in their geologic slip rates, agrees Hosgri fault. As mentioned above, geodetic and seismic data seem to support with a gradient observed in geodetic data (Feigl et al., 1990; Shen and Jackson, some right-lateral slip on the faults within the basin. However, we also consider 1993) and reflects internal shortening of the Santa Maria Basin to accommodate it likely that vertical-axis rotation of fault blocks within the Santa Maria Basin, rotation of the western Transverse Ranges relative to the nonrotating southern through variations in dip-slip displacement along strike of the reverse faults, Coast Ranges. The fan-like orientations of variable west- to northwest-striking is accommodating some of the southward decrease in right-lateral displace- faults and folds and the presence of fault-propagated folding in the Santa Maria ment on the Hosgri fault, as proposed by Sorlien et al. (1999). In addition, we Basin mimic older structures present to the east of the Little Pine fault along the hypothesize that some displacement across the northern Hosgri fault may northern edge of the western Transverse Ranges. Those structures accommo- have been transferred eastward to the West Huasna–Foxen Canyon–Little Pine dated rotation through rotational folding and variations in dip-slip displacement fault at a point north of the Santa Maria Basin during mid-Miocene to early along strike of reverse faults from late Miocene to possibly Pleistocene time Pleistocene time. This eastward transfer of right-lateral shear would have been (Onderdonk, 2005). This style of deformation likely also occurred in the Santa accommodated by the large amount (tens of kilometers) of shortening that Maria Basin during the later stages of western Transverse Ranges rotation (6 Ma occurred across the western Big Pine–Pine Mountain fault (Fig. 1) during this to present), and continued into late Pleistocene time. time period (Onderdonk, 2003). Shortening across the Santa Maria Basin may also be accommodating a decrease in right-lateral slip southward on the Hosgri fault and thereby trans- ferring some southern Coast Ranges right-lateral shear into western Transverse ■■ CONCLUSIONS Ranges shortening. Estimates of total displacement on the Hosgri fault vary from 10.5 km (Sorlien et al., 1999) to 80 km or more (Hall, 1975; Dickinson et The Orcutt Formation represents deposition on a broad, flat peneplain al., 2005; Langenheim et al., 2013). Sorlien et al. (1999) reconstructed Miocene between 119 ka and 85 ka. The Orcutt Formation was subsequently deformed horizons in the offshore part of the Santa Maria Basin and calculated 3.5 km of by folding at the surface above blind thrust faults that underlie the Casmalia right-lateral displacement across the Hosgri fault, with an additional 7 km of and Purisima Hills. Post–Orcutt Formation displacement on a deep south-dip- right-lateral shear accommodated by folding and thrust faulting that facilitate ping detachment ranges from 721 to 741 m since 119 ± 8 ka at a slip rate of small amounts of clockwise vertical-axis rotation of individual fault blocks 5.6–6.7 mm/yr. Rock uplift along the crests of the Casmalia and Purisima Hills in the Santa Maria Basin. Colgan and Stanley (2016) noted that because the ranges from 0.9 to 4.9 mm/yr. Our data show that the western Santa Maria Hosgri fault terminates offshore of Point Arguello, the 80 km of right-lateral Basin is actively inverting along major faults with high rates of displacement displacement on the Hosgri fault proposed by previous studies would have to not previously documented. This Quaternary uplift and shortening are most be absorbed by a similar amount of shortening across the Santa Maria Basin. likely due to ongoing vertical-axis rotation of the western Transverse Ranges This much shortening has not been documented in geologic reconstructions and transfer of right-lateral slip on the southern Hosgri fault to north-south of the area (Namson and Davis, 1990; Clark, 1993). Colgan and Stanley (2016) shortening in the western Transverse Ranges. presented data that show the geologic correlation used to infer 80 km of right-lateral displacement along the Hosgri fault is not unique and may not APPENDIX. LUMINESCENCE DATA AND SITE STRATIGRAPHY be valid, and they therefore inferred that total displacement on the southern Figures A1 through A5 show the luminescence data and site stratigraphy for each of the five Hosgri fault is most likely on the order of 10 km. sampling sites. Stratigraphic columns are shown on the left, with green circles denoting where Although our forward models were constructed to evaluate Quaternary samples were collected. Figure A1 includes a key for the stratigraphic columns. Two plots of the deformation, displacement of the Miocene horizons in the models indicates data from each sample are shown on the right—a probability density function for all aliquots run from each sample in terms of seconds of irradiation time by a beta source, with a rate of 0.088 that total shortening across the Purisima and Casmalia Hills since mid-Mio- Grays/s, and a radial plot of the same data (RadialPlotter by Vermeesch, 2009), which was used cene time is on the order of 4 km (Figs. 7 and 8). Additional post-Miocene to calculate the central age of the sample (Galbraith and Roberts, 2012).

North Slope Casmalia Site Stratigraphy and Luminescence Data

Key for Appendix 1 (Figures A1 through A5)

Sedimentary Structures Key

North Slope Casmalia Lower Probability Density

North Slope Casmalia Lower Radial Plot

(n=24) Central value = 5745 ± 136 (1σ) 6990 Dispersion = 11 % P(χ²) = 0.00

2 6000

5500 -2

5000

4512

σ/t 7 5 4%

t/σ 0 5 10 15 20 25 30

Figure A1. Luminescence data and site stratigraphy for the North Slope Casmalia site. A stratigraphic column is shown on the left with a green circle denoting where the sample was collected. Two plots of the data for the sample are shown on the right: (Top) Probability density function plot for all aliquots run from the sample in terms of seconds of irradiation time by a beta source with rate of 0.088 Grays/s, and (Bottom) a radial plot of the same data (RadialPlotter by Vermeesch, 2009) that was used to calculate the central age of the sample (Galbraith and Roberts, 2012). Sand grain size: vf—very fine; f—fine; m—medium; c—coarse; vc—very coarse. Gravel size: gran—granule; pebb—pebble; cobb—cobble; boul—boulder.

GEOSPHERE | Volume 17 | Number 3 McGregor and Onderdonk | Quaternary uplift and fault slip rates in the boundary zone Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/932/5319294/932.pdf 950 by guest on 28 September 2021 Research Paper

Rucker Road Site Stratigraphy and Luminescence Data

Rucker Rd Upper Probability Density

(n=23) Central value = 4786 ± 183 (1σ) 6715 Dispersion = 17 % P(χ²) = 0.00 6000

5500

2 5000 0 -2 4500 Rucker Rd Upper Radial Plot 4000

3507

σ/t 8 4 2%

t/σ 0 8 16 24 32 40 48

Rucker Rd Lower Probability Density

(n=10) Central value = 5361 ± 233 (1σ) 7029 Dispersion = 13 % χ P( ²) = 0.00 6500

6000

2 5500 Rucker Rd Lower 0 Radial Plot -2 5000

4500 4302

σ/t 5 4 3%

t/σ 0 6 12 18 24 30 36

Figure A2. Luminescence data and site stratigraphy for the Rucker Road site. A stratigraphic column is shown on the left with green circles denoting where samples were collected. Two plots of the data from each sample are shown on the right: a probability density function for all aliquots run from each sample in terms of seconds of irradiation time by a beta source with rate of 0.088 Grays/s, and a radial plot of the same data (RadialPlotter by Vermeesch, 2009) that was used to calculate the central age of the sample (Galbraith and Roberts, 2012). Refer to Figure A1 for key to stratigraphic column.

Lompoc Road Site Stratigraphy and Luminescence Data

Lompoc Rd Upper Probability Density

(n=16) Central value = 5973 ± 128 (1σ) 6937 Dispersion = 7.1 % P(χ²) = 0.00 6600

2 6400 6200

6000 0 5800

5600 -2 Lompoc Rd Upper 5400 Radial Plot 5200

49825000

σ/t 6 5 4%

t/σ 0 4 8 12 16 20 24 28

Lompoc Rd Lower Probability Density

(n=12) Central value = 6400 ± 168 (1σ) 7540 Dispersion = 7.9 % P(χ²) = 0.00 7200

7000

2 6800

6600 Lompoc Rd Lower Radial Plot 0 6400

6200

-2 6000 5851

σ/t 6 5 4%

t/σ 0 5 10 15 20 25 30

Figure A3. Luminescence data and site stratigraphy for the Lompoc Road site. A stratigraphic column is shown on the left with green circles denoting where samples were collected. Two plots of the data from each sample are shown on the right: a probability density function for all aliquots run from each sample in terms of seconds of irradiation time by a beta source with rate of 0.088 Grays/s, and a radial plot of the same data (RadialPlotter by Vermeesch, 2009) that was used to calculate the central age of the sample (Galbraith and Roberts, 2012). Refer to Figure A1 for key to stratigraphic column.

Graciosa Road Site Stratigraphy and Luminescence Data

Graciosa Rd Upper Probability Density

(n=24) Central value = 4724 ± 115 (1σ) 5798 Dispersion = 11 % P(χ²) = 0.00

2 5000

0 4500 Graciosa Rd Upper -2 Radial Plot

4000

34903500

σ/t 6 5 4%

t/σ 0 5 10 15 20 25 30

Graciosa Rd Lower Probability Density

(n=24) Central value = 5010 ± 99 (1σ) 5946 Dispersion = 8.4 % χ P( ²) = 0.00 5600

5400 2 5200 Graciosa Rd Lower 0 5000

4800 Radial Plot -2 4600

4400

4200 4066

σ/t 5 4 4%

t/σ 0 5 10 15 20 25 30

Figure A4. Luminescence data and site stratigraphy for the Graciosa Road site. A stratigraphic column is shown on the left with green circles denoting where samples were collected. Two plots of the data from each sample are shown on the right: a probability density function for all aliquots run from each sample in terms of seconds of irradiation time by a beta source with rate of 0.088 Grays/s, and a radial plot of the same data (RadialPlotter by Vermeesch, 2009) that was used to calculate the central age of the sample (Galbraith and Roberts, 2012). Refer to Figure A1 for key to stratigraphic column.

Dominion Road Site Stratigraphy and Luminescence Data

Dominion Rd Upper Probability Density

(n=16) Central value = 5048 ± 132 (1σ) 6430 Dispersion = 9.1 % P(χ²) = 0.00

5500 2 Dominion Rd Upper

0 5000 Radial Plot

-2

4500 4335

σ/t 8 5 4%

t/σ 0 5 10 15 20 25 30

Dominion Rd Lower Probability Density

(n=16) Central value = 5024 ± 95 (1σ) 5613 Dispersion = 5.9 % P(χ²) = 0.00 5400 2

5200 Dominion Rd Lower 0 5000 Radial Plot 4800

-2 4600

4472

σ/t 5 5 4%

t/σ 0 5 10 15 20 25 30

Figure A5. Luminescence data and site stratigraphy for the Dominion Road site. A stratigraphic column is shown on the left with green circles denoting where samples were collected. Two plots of the data from each sample are shown on the right: a probability density function for all aliquots run from each sample in terms of seconds of irradiation time by a beta source with rate of 0.088 Grays/s, and a radial plot of the same data (RadialPlotter by Vermeesch, 2009) that was used to calculate the central age of the sample (Galbraith and Roberts, 2012). Refer to Figure A1 for key to stratigraphic column.

ACKNOWLEDGMENTS Erslev, E., 1991, Trishear fault-propagation folding: Geology, v. 19, p. 617–620, https://doi.org/10 This work was funded by a National Science Foundation Award (1839301) to Onderdonk, and by .1130/0091-7613(1991)019<0617:TFPF>2.3.CO;2. Graduate Student Research grants to McGregor from the Geological Society of America and the Farris, A.C., 2017, Quantifying Late Quaternary Deformation in the Santa Ynez Valley, Santa Barbara College of Natural Sciences and Mathematics, California State University–Long Beach. We gratefully County, California [Master’s thesis]: Long Beach, California, California State University, 150 p. acknowledge Duane DeVecchio, Jeff Lee, Scott Johnston, Doug Yule, and an anonymous reviewer Feigl, K.L., King, R.W., and Jordon, T.H., 1990, Geodetic measurement of tectonic deformation for their excellent reviews and feedback. We thank Rick Behl, Nathan Eichelberger, Shannon Mahan, in the Santa Maria fold and thrust belt, California: Journal of Geophysical Research, v. 95, Alan Nunns, and Mary Rogan for their expertise and support. Very special thanks go to Johanna p. 2679–2699, https://doi.org/10.1029/JB095iB03p02679. Bradley and the Ernest E. Righetti II Ranch, Vidal Castrejon, John Bognuda and the Bognuda family, Galbraith, R.F., and Roberts, R.G., 2012, Statistical aspects of equivalent dose and error calculation and Bryn Smith for their endless support and generosity during field work. and display in OSL dating: An overview and some recommendations: Quaternary Geochro- nology, v. 11, p. 1–27, https://doi.org/10.1016/j.quageo.2012.04.020. Gray, L.D., 1980, Geology of Mesozoic Basement Rocks in the Santa Maria Basin, Santa Barbara and San REFERENCES CITED Luis Obispo Counties, California [M.S. thesis]: San Diego, California, San Diego State University, 78 p. 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